Magnetic tape systems provide convenient and low cost means for storing data. As one example, portable magnetic tape cartridges may be carried from a data storage drive of one data processing system to a storage facility, and, if needed, they may be carried to a data storage drive of the same or another data processing system. As another example, large quantities of magnetic tape cartridges may be stored in storage shelves of an automated data storage library and accessed from the storage shelves as needed to access the data.
There is a continuing desire to increase the data storage capacity of magnetic tape. One means of increasing data storage capacity is to increase the number of parallel tracks of data. However, as the number of parallel tracks are increased, the track widths, and margin for error between adjacent tracks, are correspondingly decreased. For example, magnetic tape systems have track patterns (spacing between adjacent track centerlines) ⅓ the width of older magnetic tape systems, meaning that the tracks are written narrower and that there is now little or no allowance for spacing between the adjacent tracks. Without spacing between the tracks, the tracks may be “shingled” in which a more recently written track whose width is greater than the spacing between the adjacent track centerlines partially overwrites one edge of the adjacent written track. As the result, the actual track width of that adjacent written track is now less than when it was written, and the centerline of that track has moved. Alternatively, the written track width may be too narrow for a read head, and the read head will read too much noise from between the tracks. Still alternatively, the written track width may be too wide, such that a written track overwrites too much of the adjacent track (more than shingling) such that the overwritten track cannot be read.
Tape heads are typically manufactured in thin film processes and have multiple write gaps and multiple read gaps. The write gaps (read gaps also) tend to vary slightly in width between tape heads, and, due to various edge or fringing effects, the write gaps tend to have effective widths that vary between tape heads. In many cases, the effective widths are about the same within a tape head, even though there may be variation between tape heads. To determine the width of the tape head write gap, the tape head is operated to write a track on a test tape, a magnetic fluid is placed on the test tape, and the test tape is read by an operator with a microscope to estimate the track width. A potential error situation can result, in that the operator must estimate where the actual magnetic edge is from the magnetic particles in the magnetic fluid.
The resultant estimated track width is then provided with the tape head, and, after the tape drive is assembled, the estimated track width is entered into a data base of a servo system of the tape drive in which the tape head is mounted. Additional sources for potential error comprise insuring that the correct data is provided for each head, and the head may have more or less skew than when in the tester, making the effective track width different.
Measurement of the width of a head having an unknown width has apparently been attempted by measuring against a recorded pattern of known width. In one example, Japanese patent JP200129127A appears to measure the time for a helical scan device to cross a longitudinal test track to estimate the head width, and in another example, Japanese patent JP120817A appears to measure the time for a floppy disk head to cross a helical test track to estimate the head width. The process is unworkable to measure a longitudinal track with a longitudinal recording system.